This invention relates generally to an apparatus for testing insulated electrical conductors, and more particularly to a spark tester including a hybrid bipolar-FET power oscillator circuit for improved power efficiency.
In the continuous testing of the insulation of an insulated conductor, it is now common practice to employ a high voltage sine wave AC potential at a frequency between about 500 Hz to about 5000 Hz. One method of generating this test potential is by means of a self-excited oscillator operating at the anti-resonant frequency of the high voltage transformer and the capacitance of the product under test to the test electrode. A suitable circuit for this purpose is described in my U.S. Pat. No. 4,952,880, the disclosure of which is herein incorporated by reference.
It is an object of the present invention to provide an improved oscillator circuit for use with an insulation testing apparatus that increases oscillator efficiency so as to increase power delivered to the load while eliminating the need for heat sinks or forced air cooling.
It is a further object of the present invention to provide an insulation testing apparatus that permits the high voltage output to be short-circuited without damage to circuit components and to provide rapid recovery of the high voltage potential to its preset value upon removal of the short-circuit.
In a first aspect of the present invention an oscillator circuit for use with an insulation testing apparatus includes a transformer having a primary winding and at least one secondary winding. The secondary winding is to be coupled across an external load. At least one bipolar junction transistor is employed in an oscillator loop and is coupled to the primary winding of the transformer for producing high frequency voltage. At least one field effect transistor is coupled in parallel with the bipolar junction transistor. Means for actuating the field effect transistor to conduct with the bipolar junction transistor upon excitation of the primary winding of the transformer is provided, whereby the current conducting through the primary winding is substantially shunted from the bipolar junction transistor to the field effect transistor to significantly reduce power loss otherwise occurring if the current were conducting through the bipolar junction transistor alone.
In a second aspect of the present invention an oscillator circuit for use with an insulation testing apparatus includes a transformer having a primary winding and at least one secondary winding. The secondary winding is to be coupled across an external load. First and second bipolar junction transistors are connected in a xe2x80x9cpush-pullxe2x80x9d operating mode and coupled to the primary winding of the transformer for producing a high frequency voltage. The primary winding has two ends each respective one of which is coupled to a respective one of the collectors of the first and second bipolar junction transistors. First and second field effect transistors are respectively coupled in parallel with the first and second bipolar junction transistors. Further provided is means for actuating the first and second field effect transistors to respectively conduct substantially synchronously with the first and second bipolar junction transistors upon excitation of the primary winding of the transformer, whereby the current conducting through the primary winding is substantially shunted from the bipolar junction transistors to the field effect transistors to significantly reduce power loss otherwise occurring if the current were conducting through the bipolar junction transistors alone.
Preferably, where the oscillator is employed in a push-pull configuration, the transformer includes a second secondary winding, and the actuation means includes a low voltage switch coupled to the second secondary winding for being closed upon excitation of the additional secondary winding, and a driver circuit powered via the low voltage switch for triggering the first and second field effect transistors to respectively conduct substantially synchronously with the first and second bipolar junction transistors.
The driver circuit preferably has first and second digital outputs respectively coupled to the gates of the first and second field effect transistors such that the first output of the driver circuit has an opposite digital state relative to the second output of the driver for actuating the first and second field effect transistors to respectively conduct substantially synchronously with the first and second bipolar junction transistors.
In a third aspect of the present invention, an apparatus for high voltage testing of the insulation of electrical conductors includes an electrode through which an insulated electrical conductor passes. The apparatus further includes an oscillator circuit having a transformer including a primary winding and at least one secondary winding. The secondary winding is to be coupled across the electrode and ground. At least one bipolar junction transistor is employed in an oscillator loop and coupled to the primary winding of the transformer for producing high frequency voltage. At least one field effect transistor is coupled in parallel with the bipolar junction transistor. Further provided is means for actuating the field effect transistor to conduct with the bipolar junction transistor upon excitation of the transformer windings and upon an oscillator signal in the oscillator loop reaching a predetermined voltage, whereby the current conducting through the primary winding is substantially shunted from the bipolar junction transistor to the field effect transistor to significantly reduce power loss otherwise occurring if the current were conducting through the bipolar junction transistor alone.
Preferably, the switching means includes a sample and hold circuit having a switch coupled to a charging capacitor for opening a voltage regulator loop and maintaining via the charged capacitor the supply voltage to the oscillator loop.
An advantage of the present invention is that the resistance and corresponding voltage drop across the field effect transistors is extremely low compared with the voltage drop across the bipolar junction transistors. This results in the shunting of most of the current from the bipolar junction transistors to the field effect transistors such that power losses through the transistors is almost completely eliminated.
Another advantage is that the apparatus embodying the present invention permits the high voltage output to be short-circuited without damage to circuit components and to provide rapid recovery of the high voltage potential to its preset value upon removal of the short-circuit.
These and other advantages of the present invention will become more apparent in the light of the following detailed description and accompanying figures.